Before ending this section, let us mention that, in order to analyse crossing equations, one would expand functions \(f(x,\theta _i,\bar{\theta }_i)\)  in Grassmann variables and restrict to \(B\) -invariants, which, in the case at hand are \(R\) -invariants. In this process, the factor \(\mathcal {M}\)  is turned to a \(6\times 6\)  matrix of differential operators. Details on this point are given in [1]}.
